1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving method of the liquid crystal display device, and particularly to an active matrix type liquid crystal display device that controls display in pixel units, and a driving method of the liquid crystal display device.
2. Description of the Related Art
The liquid crystal display device is now widely used in portable terminals, PC (personal computer) monitors, devices for commercial use, and digital TVs because of reduced thickness, light weight, and low power consumption of the liquid crystal display device. For TV use, in particular, the liquid crystal display device is compared with a CRT (Cathode Ray Tube) conventionally spread widely, and the liquid crystal display device still has problems in terms of dark-place contrast, response speed (moving image characteristics) and the like.
The liquid crystal display device has a structure in which light is emitted from a backlight under a liquid crystal panel, while each pixel of the liquid crystal panel functions as a shutter of the light. The liquid crystal display device cannot completely shut out light at a time of black display, and thus has contrast lowered in a dark place. As to the lowered contrast in a dark place, black luminance can be made lower than before by reducing the diameter of pigment particles of color filters, improvements of polarizing films and the like, and performing panel design such that liquid crystal molecules are aligned in an appropriate direction in an entire area within a pixel. However, light still cannot be completely blocked at a time of black display.
There is a technique of controlling the luminance of a backlight according to brightness while monitoring the luminance level of an input video signal. With CCFLs (Cold Cathode Fluorescent Lamps) widely used as backlight for liquid crystal display devices, local luminance control cannot be performed. Thus, in video in which a light part and a dark part are displayed simultaneously, adverse effect is produced on display of either the light part or the dark part.
As one of methods for improving the contrast, there is a conventionally known technique that controls luminance in pixel units by two laminated liquid crystal panels, and makes it possible to make black display up to a square of contrast of a single panel by making the two liquid crystal panels display black at a time of black display. For example, refer to Japanese Patent Laid-Open Nos. Hei 3-055592 and Hei 3-113427 for more information.
As to the response speed of the liquid crystal display device, liquid crystal molecules themselves are slow in response. There is a problem in particular of occurrence of a blur in a moving image as a result of the response being incomplete within one frame under a condition of a low gradation or a low temperature. In addition, because the liquid crystal display device is a hold type device in which a backlight illuminates at all times, and pixels continue being lit (continue holding a video signal), a blur in a moving image and a residual image are caused by the hold type display.
As a technique for improving the moving image characteristics (response speed) of the liquid crystal display device, an overdrive technique is known. In general, this overdrive technique basically monitors a gradation change by comparing a present frame and a previous frame with each other, and when a gradation change is detected, applies a voltage higher than a gradation voltage to be reached in only one frame in which the change is detected.
In order to improve the moving image characteristics, however, the hold type device needs to be changed to an impulse type device in which pixels blink. As techniques for improving the moving image characteristics, a scan backlight technique, black insertion and the like are widely known.
The former scan backlight technique turns off a backlight (or reduces light) for a specific time of one frame period in synchronism with timing of writing of a data signal. However, it is impossible to turn off the backlight in the same timing for all pixels in writing each pixel because the scan backlight technique turns on/off the backlight in units of regions, and a leakage of light from a region being lit into an unlit region is inevitable.
The latter black insertion is a technique of writing black in every other frame on a data signal. This black insertion is difficult to realize because the black insertion involves flicker and leads directly to a decrease in luminance as in controlling the luminance of the backlight.
Further, there is n-time speed driving as a technique for a better appearance of a moving image. This n-time speed driving improves response speed by increasing a normal vertical frequency 1.5 times or twice or more and also making use of overdrive. In addition, pseudo impulse driving is realized by selecting a gradation to be written in each of a plurality of fields divided within each frame.
In a case of double speed, for example, a data signal is written in a first field within one frame at a time of normal driving, and black is written in a second field, whereby an optical waveform is a sawtooth waveform, that is, an impulse type waveform.
Combinations of the techniques such as the overdrive technique, the scan backlight technique, black insertion, and n-time speed driving as described above have improved the moving image characteristics of the liquid crystal display device beyond comparison to the moving image characteristics in the past. As a result, a rate of prevalence of liquid crystal TVs, for example, has also been improved.